Recently, as a large capacity information recording technology, the research and development of high density optical recording techniques have progressed such that more information can be stored in a unit area. In an optical disk technology being commercialized now, a laser beam is focused on a disk surface to record and/or reproduce data recorded on the disk. Until now, in order to increase the density of the data, a technology has been developed in which the size of the focused laser spot is made smaller. The spot size is proportional to λ/NA, where the wavelength of the light source and the numerical aperture of the objective lens are assumed to be λ and NA, respectively. That is, the quantity of information being stored in one disk has been increased by reducing the wavelength of the light source and by enlarging the NA of the lens. Herein, if the classification is by the wavelength of the light source, the NA of the objective lens, and the capacity of the stored data in a 12-cm diameter and is written as (wavelength, NA, capacity), then in a CD it is (780 nm, 0.5, 650 MB) and in a DVD (650 nm, 0.6, 4.7 GB). Moreover, two kinds of classification have been proposed in the technology using a blue color laser beam source, which are (405 nm, 0.85, 25 GB) and (405 nm, 0.65, 20 GB). With this storage capacity it is possible to record high definition TV image data for about two hours.
However, the above-mentioned storage capacity is not sufficient, for instance, for use in security systems and in business systems such as broadcasting stations, etc., where, for instance, a capacity of 100 GB or more is required on one disk. Moreover, for image data, etc. which is required to be stored for several tens of years to about 100 years it is desired that as much data as possible be recorded on one disk considering the storage place for such a great amount of data storage media. The required capacity is from several hundreds of gigabytes to 1 terabyte or more.
However, it is thought that a further increase in the capacity by using the above-mentioned method is difficult. First of all, regarding reduction in the wavelength of the light source, there is an expectation that the development of a laser diode being light source is extremely difficult. Moreover, even if a laser diode were developed, there is an expectation that it would be difficult to maintain excellent record/reproduction quality since the disk substrate and protective film would absorb the light because the light source is an ultraviolet light. Research to increase the NA of the objective lens is proceeding now, and, for instance, a technology in which the NA is assumed to 1.8 is reported in the Japanese Journal of Applied Physics, Vol. 42, pp. 1101-1104. However, in this system, the light for recording/reproducing is not a regular propagating light but a so-called near-field light which is a light localized at the lens, so that it is necessary to have a system in which a lens moves on a disk while bringing the lens close to the disk surface and maintaining the distance between them. Such system has a remarkable similarity to a hard disk for magnetic recording and it makes exchanging a disk difficult which is an advantage of an optical disk.
According to the background mentioned above, a technique for improving the optical resolution effectively by installing some mechanism is proposed. Hereinafter, it will be called a super-resolution technology.
A super-resolution technology which uses a phase change recording film is reported in the Japanese Journal of Applied Physics, Vol. 32, pp. 5210-5213. A phase change recording film is typically used as a recording film for a rewritable disk such as CD-RW, DVD-RAM, DVD±RW, and a Blu-ray Disc, etc. However, herein, this recording material is not used for a recording layer but used for a layer to improve the optical resolution efficiently which is the same as the reproducing layer in the above-mentioned optical magnetic disk. Herein, such layer (film) will be called a super-resolution layer (film). In this technique, a phase change recording film is deposited on a read-only (ROM) disk and a part of the phase change recording layer is molten while reproducing. If the reflectance of the disk at the molten position is sufficiently high, the signal obtained from the molten position becomes dominant in the reproducing signals. That is, the molten position of the phase change film becomes an efficient reproducing light spot. Since the area of the molten position is smaller than the light spot, the reproducing light spot is reduced, resulting in the optical resolution being improved.
A recordable super-resolution technique is also proposed. For instance, in the Japanese Journal of Applied Physics, Vol. 43, pp. L8-L10, a technique to improve the recording density is reported in which a mark is recorded by irradiating a laser pulse to a disk which has both platinum oxide and a phase change recording layer using the same method as recording a mark to a regular rewritable optical disk and super-resolution reproduction is performed. In this technique, the platinum oxide expands locally by irradiating the write laser power, and the film thickness of the phase change film is modulated corresponding to the mark. While reproducing, the super-resolution effect is obtained by melting only the part where the film thickness of the phase change layer is thin. As a result, a write-once type super-resolution disk which is available to record only once is achieved.
Moreover, the Japanese Journal of Applied Physics, Vol. 37, pp. L516-L518 reported a method to realize a rewritable disk using a technique in which a phase change film described in the Japanese Journal of Applied Physics, Vol. 37, pp. 5210-5213 is used as a super-resolution film. In this technique, two kinds of phase change films are used for the super-resolution film and the recording film, and the optical absorbance of each phase change film is controlled by the film thickness, thereby, the reproducing is performed without erasing the phase change mark written in the recording film while melting the super-resolution film during reproducing. In this technique, a material is selected so as that the crystallization speed of the phase change material used for the recording layer is made slower than that of the super-resolution film. Therefore, the recording mark is made difficult to erase during reproducing, resulting in the durability against reproduction being ensured.    [Patent document 1] JP-A, No. 244870/1995    [Patent document 2] JP-A, No. 282674/1993    [Patent document 3] JP-A, No. 159357/1993    [Non-patent document 1] Japanese Journal of Applied Physics 42, 1101-1104    [Non-patent documents 2] Japanese Journal of Applied Physics 32, 5210-5213    [Non-patent documents 3] Japanese Journal of Applied Physics 43, L8-L10    [Non-patent documents 4] Japanese Journal of Applied Physics 37, L516-L518